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Each human being has 23 pairs of chromosomes ( equals
46 ) in each nucleus in the body
Number of
Eggs Sperm cells or pollen Body
cells cells cells
Hawkweed 4 4 8
Sedge 21 21 42
Hen 18 18 36
Human 23 23 46
Kangaroo 6 6 12
The structure of chromosomes
Each chromosome contains one double ­ stranded DNA
The DNA us folded and coiled so that it can be packed
into a small space
Protein called histones surround the…read more

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DNA ( stands for deoxy rilo nucleic acid ) has a twisted
double ­ heux structure like a twisted ladder
Each ` rung ' on the ladder is made of a pair of basis
There are 4 types of basis G, C, A and T
G and C always go together
A and T always go together…read more

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Sections of DNA are called genes
We inherit half our genes from our father and half our
genes from our mother
Sex cells are the only cells to contain only half the
Males are said to be XY and females XX ­ mostly because
of the shape of sex cells
The X and Y chromosomes are made up of genes which
control sexual development ( among other things )
People still cannot choose the sex of their children, but we
do know how…read more

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The egg contains twenty ­ two ` body ' chromosomes and
one sex chromosome
It can only be an X chromosome so all eggs have an X
From the father ( XY )
Sperm also have twenty ­ two ` body ' chromosomes
Half the sperm have an X chromosome and half the sperm
have a Y chromosome
If one of the X sperms reaches the egg first and
fertilizes it, the result is XX ( a girl )
If one of the Y sperms…read more

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